An imaginary line extended perpendicular to the average plane of the joint that replaces the bone when modeling osteokinematics. It is not the axis of movement although the two may coincide in which case an osteokinematic spin occurs

the","That which is unconditioned and uncaused by anything else; it is frequently thought of as God, a perfect and solitary, self-caused eternal being that is the source or essence of all that exists but that is itself beyond the possibility of conceptualization or definition",